The invention is directed to field emission displays having reduced operating voltages and methods of producing the same. In addition, the invention is directed to field emission displays having reduced threshold voltages and methods of producing the same.
Cathode ray tube (CRT) displays, such as those commonly used in desk-top computers screens, function as a result of a scanning electron beam from an electron gun, impinging on phosphors of a relatively distant screen. The electrons increase the energy level of dopant(s) in the phosphors. When the dopant(s) return to their normal energy level, they release energy from the electrons as photons of light, which is transmitted through the glass screen of the display to the viewer.
Field emission displays seek to combine cathodoluminescent-phosphor technology with integrated circuit technology to create thin, high resolution displays wherein each pixel is activated by its own electron emitter. Flat panel display technology is becoming increasingly important in appliances requiring lightweight portable screens. Currently, such screens use electroluminescent, liquid crystal, or plasma display technologies. A promising technology is the use of a matrix-addressable array of cold cathode emission devices or field emission devices (xe2x80x9cFEDsxe2x80x9d) to excite pixels of phosphors on a screen. These devices are generally comprised of a baseplate and a faceplate. The faceplate has a cathodoluminescent phosphor coating that receives a patterned electron bombardment from an opposing baseplate thereby providing a light image which can be seen by a viewer. The faceplate is separated from the baseplate by a vacuum gap, and outside atmospheric pressure is prevented from collapsing the two plates together by physical standoffs between them, often referred to as spacers. Arrays of electron emission sites (emitters) are typically sharp cones that produce electron emission in the presence of an intense electric field. In the case of most field emission displays, a positive voltage is applied to an extraction grid relative to the sharp emitters to provide the intense electric field required for generating cold cathode electron emissions. Typically, FEDs are operated at anode voltages well below those of conventional CRTs.
The faceplate of a field emission display operates on the principle of cathodoluminescent emission of light. A color image can be obtained using a color sequential approach sometimes referred to as spatial integration. Nearly all commercially successful color displays today employ spatial integration to provide a color image to the viewer. A common way to employ spatial integration is to provide red, green, and blue pixels which are addressed in the form of R/G/B triads. The intensity of each of the color dots within the triad is adjusted relative to one another to produce a range of colors within the triangular boundary formed by the color coordinates of the R,G, and B dots as depicted on the 1931 or 1976 C.I.E. chromaticity diagram. The human eye is then relied upon for integrating the spatially separated R/G/B dots into a perceived color image.
Spatial color displays generally employ a black region separating the red, green, and blue patterned dots. A major advantage of the black region, referred to as the black matrix, is to improve the contrast of the display in ambient light. When a black matrix is employed on the faceplate it absorbs ambient incident light, thereby improving the contrast performance of the display. (See, e.g., U.S. Pat. Nos. 4,233,623 and 4,891,110 both incorporated herein by reference.)
As stated above, in field emission displays, electrons are emitted toward the phosphor coated screen. A phosphor is generally a substance, either organic or inorganic, liquid or crystalline, that is capable of luminescencing, i.e., of absorbing energy from sources such as x-rays, cathode rays, ultraviolet radiations, alpha particles and emitting a portion of energy in the ultraviolet, visible or infrared. Examples of such phosphors include: oxides, halides, silicates, borates, sulfides, titanates, phosphates, halophosphates, tungstates, germanates, stannates, indates, aluminates, gallates, arsenates, germinates, vanadates of zinc, silver, cadmium, indium, zirconium, germanium, tin, lead, strontium, titanium, lithium, sodium, potassium, thallium, gallium, magnesium, strontium, calcium, barium, thorium, scandium, yttrium, vanadium, and the Lanthanide Series rare earth metals. It should be noted, as known to one of ordinary skill in the art, not all of the metal ions listed above will form compounds with all of the listed anionic groups and in some cases not all of the formed compounds will be useful as lattices for phosphor preparation. However, all phosphors are not recommended for use in FEDs because the cathodes are in very close proximity to the faceplate and are sensitive to any electronegative chemicals arriving on the cold cathode emitter surfaces which could absorb and increase the value of the work function. Consequently, sulfides of cadmium or zinc are not recommended for use in FEDs. Particularly preferred phosphors for use in FEDs include ZnO:Zn, Y3(Al,Ga)5O12:Tb, Y2SiO5:Ce, Y2O3:Eu, Zn2SiO4:Mn, ZnGa2O4:Mn. These phosphors tend to be dielectric in nature (except for, e.g., ZnO:Zn and ZnGa2O4) and consequently, the threshold voltages (the voltage necessary to excite the phosphor) tend to be high. For example, generally the threshold voltages for phosphors utilized in FEDs tend to range from about 500 volts to about 2000 volts. Generally, the threshold voltage of phosphors in vacuum, fluorescent displays has been lowered by adding conducting materials to the phosphors before application to the screen, such as non-luminescent zinc oxide or indium tin oxide powders. (See, e.g., xe2x80x9cProperties of ZnO-Containing Phosphors Under Low Voltage Cathode Ray Excitation,xe2x80x9d H. Hiraki, A. Kagami, T. Hase, K. Narita, and Y. Mimura, Journal of Luminescence, 12/13 (1976) p. 941-946 which is incorporated herein by reference). It has been found that in operation, a charge builds up on the phosphors which are not conductive or semi-conductive. The incident electrons on the phosphors surface are reflected, scattered, or absorbed by the phosphor. Furthermore, if the energy of these incident electrons is greater than a few tens of eV, then they can create a large number of secondary electrons within the phosphor screen. Some of these secondary electrons can escape back into the vacuum provided they have sufficient energy to overcome the work function of the phosphor surface. This can lead to the floating surface of the phosphor to shift its potential when the number of incident electrons is not equal to the number of secondary electrons escaping from the surface. The negative charge built up on the phosphor screen, by reducing its potential, seriously diminishes the light output, leading to an unstable emission.
Due to the fact that field emission displays will become important in portable appliances which rely on portable power sources, there is a need to reduce the threshold and operating voltages of such devices. This reduction in voltage is also important because of the small distance between the emitter and the faceplate which can lead to arcs, if the voltage is too high. The present invention provides field emission displays with reduced threshold voltages and thereby lower operating voltages. In addition, the present invention provides a method of manufacturing displays with reduced threshold voltages and operating voltages.
The invention is directed to a field emission display comprising: (1) a baseplate comprising an electron emitter cathode, and (2) a faceplate anode having phosphors and a matrix coated thereon, wherein said matrix lowers the threshold voltage of the display.
The invention is also directed to a reduced operating voltage field emission display comprising: (1) a baseplate comprising an electron emitter, and (2) a faceplate comprising a screen, phosphors on said screen, and a matrix comprising conductive or metallic particles around said phosphors.
The present invention is further directed to a field emission display comprising: (1) a baseplate for emitting electrons, and (2) a faceplate screen having phosphors thereon, said phosphors surrounded by a black matrix wherein said matrix reduces the threshold voltage of the display and thereby also reduces the operating voltage.
In addition, the present invention is directed to a process for making a field emission display screen having a faceplate substrate comprising: (1) coating the substrate with a conductive layer; (2) coating the substrate with photoresist; (3) patterning the photoresist; (4) depositing a matrix comprised of conductive or metallic particles on the substrate; and (5) removing the photoresist.